Linear hall circuit for measuring magnetic field strength

ABSTRACT

A linear Hall circuit that utilizes a single-ended output Hall current to sense a magnetic field so as to measure the magnetic field strength. A first operational amplifier configured as a transresistance amplifier converts the output Hall current of a Hall device to an output voltage. To avoid inducing a bias-based nonlinearity in the output Hall current, a second operational amplifier, having two resistors connected to both bias electrodes of the Hall device, respectively, is used to inject feedback Hall currents in them. The average value of the bias currents is then kept from varying with the output Hall current of the Hall device.

FIELD OF THE INVENTION

The present invention relates to a magnetic sensing circuit formeasuring magnetic field, and more specifically, to a Hall circuit formeasuring magnetic field strength.

BACKGROUND OF THE INVENTION

Recently, the study of Hall device techniques has been developed,resulting in the discovery of the current mode of Hall device. Forexample, as shown in FIG. 1, European patent No. EP0357013 discloses acircuit which employs a method of injecting a current into a Hall axisof a Hall device (HD). In this circuit, the injected current is equal tothe output Hall current Ih_(o) and is then converted to an outputvoltage indicating the magnetic field strength so that the magneticfield can be measured. However, it is due to the presence of the Hallcurrent that the currents at the two bias contacts of the Hall devicemay not be equal to each other and may vary with changes in the externalmagnetic field. This leads to nonlinearity between the Hall current andthe magnetic field to be measured and consequently results in inaccuratemeasurements of the magnetic field. Furthermore, there also is anotherproblem which affects measurement in the circuit. The zero-point of theoutput voltage thereof is at a value of a portion of the voltage of thepower supply divided by the internal resistance of the Hall device andthe bias resistors. Therefore, any discreptancy in the internalresistance of the Hall device and any shift of temperature will resultin the offset of the output zero-point of the circuit. The symmetricalbias solution presented by Chinese Patent Application. No. 90106846.2prevents the sum of the currents in the two bias contacts of the Halldevice from changing with that of magnetic field so as to provide alinearity between the Hall current and the magnetic field to bemeasured. The output zero-point of the circuit takes the middlepotential of the power-supply voltage of the circuit, and is independentof the internal resistance of the Hall device. Thus the circuit is keptrelatively steady. However, in this circuit, since the output zero-pointis a floating point, when positive and negative dual-power supplies areemployed, any change of the voltage of the power supplies will cause ashift in the output zero-point accordingly.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide a Hallcircuit which has a steady reference zero-point, e.g. ground potentialand a good linearity between the Hall current and the magnetic field tobe measured so as to present an accurate magnetic field measurement.

According to the invenion, a Hall circuit for measuring magnetic fieldcomprises a Hall device and a bias means connected to a first and asecond bias contacts of the Hall device, wherein a current injectionmeans is connected to both bias contacts of the Hall device to force theoutput of the Hall axis of the Hall device to be kept at a predeterminedpotential, which wilt then be used as an output zero-point of thecircuit, an amplifier means is connected to the output of the Hall axisof the Hall device, said amplifier means converts the Hall currentoutput from the Hall device into the output voltage of the circuitduring the measurement of the magnetic field to indicate the measurementof the magnetic field.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following the invention will be described in detail in connectionwith examples shown in the accompanying drawings, in which:

FIG. 1 shows a circuit arrangement of the prior art;

FIG. 2 is a diagram of the Hall circuit according to the invention;

FIG. 3 shows an equivalent circuit of FIG. 2; and

FIG. 4 shows an embodiment of the Hall circuit according to the presentinvention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 2 presents a preferred embodiment of the Hall circuit according tothe invention. The Hall circuit of the invention comprises a Hall deviceas shown by the box 15 having a first and a second bias contacts 15a and15c connected to a first and second current sources (I₁) 11 and (I₂) 21respectively, and a first and a second Hall current contacts 15b and 15dconnected to the negative inputs 14 and 16 of a first and a secondoperational amplifiers (OA1 and OA2) 17 and 25 respectively. Thepositive inputs of both amplifiers are connected to the reference ground27 so that the output voltage V_(O) at the output 31 of the secondoperational amplifier 25 takes this reference ground 27 as its ground.Since there is no current at the reference ground 27, this point can beused as a setting or adjusting terminal for the zero-point of the outputwithout involving any additional offset. The output of the firstoperational amplifier 17 is connected to both first and second biascontacts 15a and 15c of the Hall device 15 via feedback resistors 13 and19 (R1 and R2) respectively. Therefore, the output of the firstoperational amplifier 17 injects the feedback currents Ih₁ and Ih₂ intothe first and second bias contacts 15a and 15c of the Hall device 15 viathe resistors R1 and R2 when a magnetic field to be measured induces aHall current Ih in the Hall device as shown in FIG. 3 by the Hallcurrent source 15e. This negative feedback connection will keep thefirst Hall contact 15b at the ground potential.

When the feedback resistors R1 and R2 are equal, the injected currentsIh₁ and Ih₂ fed to the first and second bias contacts 15a and 15c of theHall device 15 are equal to each other too. Therefore, the sum of thecurrents of both bias contacts 15a and 15c of the Hall device 15 isindependent of the magnetic field so that the linearity between the Hallcurrent Ih and the magnetic field to be measured is ensured. Since bothIh₁ and Ih₂ are equal to Ih, (they may be referred to as "feedback Hallcurrent") and have a linear relation to the magnetic field to bemeasured, consequently the output Hall current Ih_(o) of the Hall device15 as well as the output voltage V_(O) of the circuit also have a linearrelation to the magnetic field being measured. In the invention, thebias current sources may be replaced by temperature-dependent currentsources or thermal resistors, and this may achieve temperaturecompensation easily.

The response of the circuit of FIG. 2 to the magnetic field will befurther described in the light of FIG. 3. FIG. 3 is a simplified diagramof the circuit of FIG. 2, wherein the Hall device is represented by anequivalent bridge with four equal arms as shown within dashed lines, theHall current generated under the effect of a magnetic field isrepresented by a Hall current source Ih (15e) on the diagonal line ofthe Hall axis. Furthermore, the bias current sources I₁ and I₂ areomitted in FIG. 3 for clarity because they do not affect the signalamplifying operation in the circuit. It is obvious in the light of FIG.3 that since the Hall device is in fact a bridge with four equal arms,the feedback current Ih₁ is equally divided between the two upper armsof the bridge. Likewise, Ih₂ is equally divided between the two lowerarms of the bridge. As there is no current coming in and going out ofthe Hall contact 15b, the equality (Ih₁ +Ih₂)/2=Ih is established.Therefore, the output Hall current Ih_(o) at the second Hall contact 15dwill be:

    I.sub.out =Ih.sub.1 +Ih.sub.2 =2Ih

This output Hall current Ih_(o) is converted into the final outputvoltage V_(O) by the current-voltage converter composed of the secondoperational amplifier 25 and the feedback 29 to indicate the magneticfield strength.

FIG. 4 shows a real circuit according to the principle of the invention,where the bias current sources I1 and I2 of FIG. 2 are replaced with tworesistors R1, R4 connected to the power supplies V+ and V- respectively,and wherein the parameters of each element in the circuit are asfollows: R1=R2=R3=R4=7.5 KΩ; R5=100 KΩ; the first and second operationalamplifiers 17 and 25 are μA 741 respectively; the Hall device 15 is aGaAs epitaxial device with a sensitivity of 0.4/Tesla; voltage sourcesare V₊ =+15 V, V₃₁ =-15 V respectively; the sensitivity of the outputvoltage V_(O) in response to the magnetic field is about 80 V/Tesla.

Thus, the present invention provides a Hall circuit with a steadyreference zero-point and an advantageous linearity between the outputvoltage and the magnetic field to be sensed.

While the invention has been described in conjunction with specificembodiments thereof, it is evident that many alterations, modifications,and variations will be apparent to those skilled in the art in light ofthe foregoing description. Accordingly, it is intended to embrace allsuch alterations, modifications, and variations in the appended claims.

What we claim is:
 1. A circuit for measuring magnetic field, comprising:a Hall device (15) having first and second bias contacts (15a, 15c) and first and second Hall contacts (15b, 15d); bias means (11, 21) connected to said first and second bias contacts (15a, 15c) of said Hall device (15); a first amplifier means (13, 17, 19, 27) having an inverted input (14) connected to said first Hall contact (15b) of said Hall device (15) and an output connected to said first and second bias contacts (15a, 15c) for injecting a first and second feedback Hall current (Ih1, Ih2) respectively to said first and second bias contacts (15a, 15c) of said Hall device (15); a second amplifier means (25, 27, 29) having an inverted input (16) connected to said second Hall contact (15d) of said Hall device (15) for receiving an output Hall current (Ih_(o)) of said Hall device (15) and for conveying it to an output voltage of said circuit.
 2. A circuit according to claim 1, wherein said first and second Hall contacts (15b, 15d) of said Hall device (15) are forced at a common reference ground (27) by means of a feedback circuit configuration of said first and second amplifier means.
 3. A circuit according to claim 2, wherein said second amplifier means comprises a transresistance amplifier composed of an operational amplifier (25) and a feedback resistor (29) connected between the second amplifier output (31) and said second amplifier inverting input (16) thereof, and wherein a non-inverting input of said amplifier (25) is connected to said reference ground (27).
 4. A circuit according to claim 1, wherein said output Hall current (Ih_(o)) of said Hall device (15) is a substantially linear function of a magnetic field applied to said circuit.
 5. A circuit according to claim 1, wherein said first and second feedback Hall currents (Ih1, Ih2) are substantially equal to each other and the sum of said feedback Hall currents are substantially equal to said output Hall current (Ih_(o)) of said Hall device (15).
 6. A circuit according to claim 1, wherein said first amplifier means is a transconductance amplifier comprising a first operational amplifier (17) and first and second feedback resistors (13, 19); wherein said inverting input (14) of said operational amplifier (17) is connected to said first Hall contact (15b) of said Hall device (15) and a non-inverting input of said amplifier (17) is connected to said reference ground (27); said first feedback resistor (13) is connected between the output of said operational amplifier (17) and said first bias contact (15a) of said Hall device (15); said second feedback resistor (19) is connected between the output of said operational amplifier (17) and said second bias contact (15c) of said Hall device (15); and said two feedback resistors (13, 19) have substantially the same resistance value.
 7. A circuit according to claim 1, wherein said bias means comprises two current sources (I1, I2) which provide substantially equal current values respectively.
 8. A circuit according to claim 1, wherein said bias means comprises two resistors (R1, R4) which have substantially equal resistance values respectively. 